The present invention relates, in general, to non-linear spectroscopy systems and, in particular, to a system and method for simultaneously measuring the amplitude and phase of second harmonic radiation over a broad spectral range without laser tuning.
The continual demand for enhanced integrated circuit performance has resulted in numerous advancements in semiconductor processes. One example of such advancement is a considerable scaling down of semiconductor process feature sizes. While such scaling has improved certain performance aspects, it has also created a number of challenges in areas such as fine feature measurement and characterization during production. Spectroscopy systems and techniques are widely used to provide detail measurement and characterization in such applications.
Spectroscopy systems are commonly used to non-invasively measure the properties or states of a semiconductor surface or sub-surface interface by analyzing light reflected therefrom. A typical objective of spectroscopy is the measurement of phase shift in the light reflected from the material under examination. Spectroscopy systems generally fall within one of two categories: linear spectroscopy or non-linear spectroscopy. Linear spectroscopy systems and non-linear spectroscopy systems typically differ in their ability to detect and characterize phenomena and material properties of particular interest; and are therefore generally employed in different applications.
Linear spectroscopy systems are typically characterized by operation involving a single wavelength of light. Linear systems typically use lamps (e.g. incandescent or arc lamps) as a light source; and typically examine a sheath-like area around the surface of interest. Conventionally, linear systems are employed to measure bulk properties of some material, such as film thickness or a varying chemical composition. For example, in a typical semiconductor processing application, a linear system would measure thickness and composition of a epitaxial growth film.
In comparison, non-linear spectroscopy systems are typically characterized by operation involving varying wavelengths of light. Typically, a high intensity light source, such as a laser, is used. Non-linear systems possess unique diagnostic capabilities due, in part, to a high surface specificity and sensitivity. This means that the reflection point from the item under examination is confined to one or two atomic layers in the immediate area of a surface or sub-surface interface.
This comparison is illustrated with reference now to FIGS. 1a and 1b. FIG. 1a provides a representative illustration of a linear spectroscopy system, specifically a spectroscopic ellipsometer 100. In ellipsometer 100, light 101 emitted from source 102 (typically an incoherent white light source such as a Xenon arc lamp) is filtered through a polarizing element 104, and directed by focusing element 106 (typically a lens) at a target sample 108 (e.g. SiGe) within processing unit 110 (e.g. a Chemical Vapor Deposition chamber). Light 101 is reflected from sample 108 through a second element 106, passing through analyzer element 110 (e.g. a polarization analyzer) to detector 114; where data such as the amplitude of the reflected light is evaluated.
FIG. 1b provides a representative illustration of a second harmonic (SH), non-linear spectroscopy system 150. In system 150, light 152 emitted from source 154 (typically a laser), with frequency 156 of xcfx89 is filtered through polarizing element 158, and directed by focusing element 160 (typically a lens) at a target sample 162 (e.g. SiGe) within processing unit 164 (e.g. a Chemical Vapor Deposition chamber). Light 152 is non-linearly reflected from sample 162 through a second element 160, passing through analyzer element 166 (e.g. a polarization analyzer) to detector 168; detecting light at twice the incident frequency (2xcfx89) 170 created by the non-linear reflection. Detector 168 evaluates data such as the amplitude of the reflected light.
Thus, the spectra of the reflected light is used to measure material properties. System 100 detects light at the same optical frequency as the incident light reflected from the sample. System 150 detects light at twice the incident frequency created by non-linear reflection. System 100 measurement therefore characterizes bulk properties of a sample, such as thickness and average composition of an epitaxial film; while system 150 measurement characterizes properties of the interface or surface of the film, such as surface composition, interface dc electric fields, and atomic and molecular adsorption, which can used to evaluate growth chemistry and rates. This information is of particular interest in semiconductor processing.
For semiconductor processing applications, spectroscopy system users are typically interested in characterizing the interface between a growing film and its substrate; for purposes such as detecting the presence of contamination or improper bonding, or measuring material strain. As previously presented, linear reflected light is insensitive to such phenomenon; compelling users to either employ non-linear systems or other alternative methods of measurement and characterization.
This presents a dilemma, however, because spectroscopy users need systems that are compact, inexpensive, simple to use. Conventional linear systems often possess these characteristics, making them a viable choice for use in high volume commercial production. Previously, non-linear spectroscopy systems have not been commercially viable, and thus limited to research and academic applications, because nobody has been able to produce them in a compact, simple to use, or inexpensive manner. Therefore, conventional commercial spectroscopy systems are typically linear in nature. For example, linear systems such as spectroscopic ellipsometers are widely used in the semiconductor industry.
Further, conventional non-linear systems are difficult to use in certain spectroscopic modes; particularly in sampling different wavelengths of second-harmonic (SH) reflected light. To examine varying wavelengths of SH reflected light, conventional systems require a manual tuning process. This results in a serial mode of data acquisition (i.e. sampling one wavelength at a time); which decreases the speed and efficiency of such systems, and increases their complexity of use. Additionally, conventional non-linear systems typically are limited in phase shift measurement; again requiring serial mode data acquisition and complex, time-consuming movement of system apparatus.
From the foregoing, it is recognized that a need has arisen for commercially viable non-linear spectroscopy systems and methods. Further, a need has arisen for a non-linear spectroscopy system, providing simultaneous measurement of both amplitude and phase of second harmonic radiation over a broad spectral range without requiring superfluous system tuning or apparatus adjustment; and further providing parallel mode data acquisition, acquiring multiple wavelengths simultaneously, and increasing system speed and efficiency while overcoming the aforementioned limitations of conventional methods.
In the present invention, a frequency domain interferometric second harmonic (FDISH) spectroscopy system is provided for use in high volume commercial applications, such as semiconductor processing; providing non-linear (i.e. second harmonic) spectroscopy systems having unique diagnostic potential due (at least in part) to unusually high surface specificity and sensitivity; providing real time measurement during processing, parallel mode data acquisition, and a unique method of acquiring phase shift information in second harmonic radiation.
In one embodiment of the present invention, a method of spectroscopically analyzing amplitude and phase information of a particular sample comprises providing a femtosecond laser source positioned in an angularly distal relationship to the sample, generating from the laser source a primary light pulse of substantial peak intensity and spectral bandwidth directed at the sample, and providing a reference medium interposed between the light source and the sample, fixed in position with respect to the sample. A portion of the primary light pulse is directed through the reference medium generating a reference second harmonic signal directed at the sample, which propagates collinearly with the primary light pulse towards the sample. A spectrometer is provided, positioned in an angularly distal relationship to the sample and opposing the laser source, to receive second harmonic reflections of the primary pulse and reference signal from said sample. The second harmonic reflections received are then analyzed as desired by a user.